It will briefly be recalled that a tire having a radial carcass reinforcement comprises, in a known manner, a tread, two inextensible beads, two sidewalls joining the beads to the tread and a belt placed circumferentially between the carcass reinforcement and the tread, this belt consisting of various plies (or “layers”) of rubber which may or may not be reinforced by reinforcing elements (or “reinforcements”) such as cords or monofilaments, of the metal or textile type.
The tire belt generally consists of at least two superposed belt layers or plies, sometimes known as “working” plies or “crossed” plies, the reinforcements of which are in practice arranged parallel to one another within a layer, but crossed from one layer to the other, that is to say inclined, whether symmetrically or not, to the median circumferential plane, by an angle which is generally between 10° and 45° depending on the type of tire in question. Each of these two crossed layers consists of a rubber matrix generally based on isoprene, sometimes known as “calendaring gum” that coats the reinforcements. The crossed layers may be finished off by various other auxiliary rubber plies or layers, having widths that vary depending on the case, and which may or may not contain reinforcements; mention will be made by way of example of simple rubber pads, of layers known as “protective” layers, the role of which is to protect the rest of the belt from external attack, perforations, or else layers known as “hoop reinforcement” layers comprising reinforcements oriented substantially along the circumferential direction (layers known as “zero degree” layers), whether they are radially external or internal compared to the crossed layers.
This tire belt must meet, in a known manner, numerous, sometimes contradictory, requirements, in particular:    (i) to be as rigid as possible at low deformation, as it substantially contributes to stiffening the tire crown;    (ii) to have as low a hysteresis as possible, to on the one hand, minimise overheating of the internal zone of the crown when rolling and, on the other hand, to reduce the rolling resistance of the tire, which is synonymous with fuel economy; and    (iii) finally to have a high endurance, in particular with respect to the phenomenon of separation or cracking of the ends of the crossed layers in the “shoulder” zone of the tire, a problem which is known by the term “cleavage”.
The third condition especially requires that rubber compositions incorporated into the formation of tire belts have a very high resistance to crack propagation and to thermal oxidation, in particular achieved due to the use of antioxidant agents that offer an effective anti-ageing protection.
This requirement is particularly high for the tire covers of heavy vehicles, which are designed to be able to be retreaded one or more times when the treads that they comprise reach a critical degree of wear after prolonged rolling.
The antioxidants used for a very long time as anti-ageing protective agents in rubber compositions for tires, especially in the belts of such tires, belong to the family of the derivatives of para-phenylenediamine (PPD) such as for example N-isopropyl-N′-phenyl-para-phenylenediamine (I-PPD) or N-1,3-dimethylbutyl-N′-phenyl-para-phenylenediamine (6-PPD) which are simultaneously excellent antioxidants and antiozonants (see, for example, Applications WO 2004/033548, WO 2005/063510, WO 2005/133666).